Such large-format fireclay refractory bricks serving for a lay out of the steel construction of a tin bath of flat glass facilities and being made from a material of the system Al.sub.2 O.sub.3 --SiO.sub.2, are known from DE 42 06 734 C2. The here interesting part of the glass producing facility has a steel construction in its lower region, the steel construction being laid out with large-format fireclay refractory bricks. Thereby a production tub is formed being filled with liquid tin. The melted glass is poured out on the surface of the tin bath. The glass expands on the surface of the tin bath and is drawn as a thin band with substantial width over the surface of the tin bath. This is the known method of producing flat glass according to the float technique. The flat glass contains about 15% Na.sub.2 O. Na.sub.2 O diffuses into the melted metal at the contacting surface between the glass and the liquid tin. In the tin bath sodium and atomic oxygen appear in a solved manner. The solubility of sodium and atomic oxygen in tin is a function of the temperature. Producing flat glass temperatures of about 1200.degree. C. to 600.degree. C. occur in the flow direction of the glass. Parts of the tin containing sodium contact the surfaces of the blocks of the fireclay refractory bricks due to thermically and mechanically induced floating of the liquid tin in the tin bath. Thereby an exchange of sodium occurs between the tin and the fireclay refractory bricks. The atomic sodium penetrates into the refractory material and reacts first on the siliciumdioxide of the glass phase of the fireclay refractory bricks generating sodiumoxide. Due to the reduction of phases containing sodiumdioxide the reduced parts of the fireclay refractory bricks show a grey to black colour.
The known fireclay refractory bricks being used in such glass producing facilities may have a length of 1000 mm, a width of 600 mm and a height of 300 mm. They are made from grains of different grading, clay and alkalialumosilicate. After firing mainly mullite, a little amount of cristoballite, and a glass phase are present. The amount of the glass phase of such a fireclay refractory brick is determined by the amount of sodiumoxide and potassiumoxide. These oxides substantially influence the chemical composition of the glass phase. This chemical composition is important for the forming of the kind of alumosilicate--nepheline or albite--during the presence of metallic sodium in layers close to the surface of the fireclay refractory brick. The thermical expansion factor of nepheline is about four times as much as the factor of mullite. This results in an enlargement and in a growing of the layers of the fireclay refractory brick being adjacent to the surface contacting the tin bath. Thereby these layers contact each other due to the rectangular shape of the bricks. Tension occurs.
On the other hand it is necessary to maintain the gaps between the grinded side surfaces of the fireclay refractory bricks as small as possible and thereby sealed in order to prevent the downward passage of the melted tin. Otherwise this would result in a damage of the steel construction holding the fireclay refractory bricks. Since the penetrating of the liquid tin in the gap cannot be avoided in all cases it is the steel construction which is cooled in order to freeze the liquid tin.
Due to the increase of volume of the layers of the bricks contacting the tin bath chipping off occurs with respect to these layers generating at the corners and edges of the surfaces contacting the tin bath. Since the ceramic material of the fireclay refractory bricks has a lower relative density than the tin floating of the chipped off material from the bricks within the tin bath in upward direction occurs. This may cause substantial trouble in the production of the flat glass.